In a typical water or waste water treatment plant, solid objets are removed from the liquid flowing through a fluid channel using one or more removal devices. Commonly, the first such device used to remove solids (also known as screenings) is a bar screen (also known as a bar rack) made up of a matrix of parallel bars placed vertically across the channel. The screen acts as a sieve to trap solids larger than the clear space between adjacent bars. Typical solids are pieces of paper, plastics and the like, and also somewhat larger objects.
The screen must be cleaned frequently of the accumulated solids. In small plants, a person using a rake or similar device scrapes the solids from the screen periodically. In larger plants, the raking operation is performed mechanically.
The screen may be may be cleaned by a rake moving in the upstream side of the screen (front cleaning) or by a rake moving in the downstream side of the screen (back cleaning). Because the solids are trapped from the upstream side of the screen, a front cleaning rake may have relatively short teeth which project into the spaces between the bars, while a back cleaning rake must have relatively longer teeth that extend through the bars into the upstream side.
An advantage of a back cleaned screen is that the rake is less likely to be jammed by a large object such as a rock, piece of pipe, or hose nozzle than a front or upstream cleaned rake would be. This is because a front cleaned rake may directly impact an object stuck in the channel, while only the teeth, at most, of a back cleaned rake will impact such an object. A disadvantage of the back cleaned rake is that for a given screen, the relatively long teeth of a back cleaned rake are less robust and more susceptible to bending that the shorter teeth of a comparable front cleaned rake.
Many different devices have been developed to mitigate the effects of jamming for a front cleaned screen. Examples of such devices are electric current sensing components which react to a sudden change in the motor current demand, direct shaft torque sensing, and more recently, sophisticated systems of flexible linkages, preferably equipped with bearings to prolong the components lifetime. Such devices may also be useful in a back cleaned screen. While these devices may be effective, they are in general complex, some are expensive, and some require a significant amount of space. Their utility in smaller bar screens has thus been very limited.
It can be appreciated in light of the foregoing discussion that there exists a need for a front cleaned rake that is protected from damage from impacting a stuck object that is simpler, more reliable, and more economical than known devices. Such a device is particularly useful in providing the advantages of a front cleaned rake than (e.g., shorter teeth) while eliminating the disadvantages. Without limiting the advantages of the present invention to the foregoing, it is a purpose of the present invention to provide such a device.